Rigging convention for limb joint dominant axis?
In my rigs, the dominant rotation axis is always X.

It's consistent with Rigify's convention, and less a hassle when using X-Axis Mirror for faster alignment. Could be confusing, though, because visually it's always the Z axis that looks mirrored. So I'd just pay attention to the Z axis when orienting the bones.



For translation bones (like sliders) I use the Z axis, also because of the way X-Axis Mirror aligns symmetrical bone rolls. That is, if the animators use Gimbal transform orientation, they could focus on using the red (X axis) part of the rotation widget, and blue (Z axis) part of translation widget.